1. Field of the Invention
This invention relates in general to nuclear logging and in particular to a new and improved pulsed neutron logging method and apparatus for determining the thermal decay time constant .tau..sub.F and correlative capture cross-section .SIGMA..sub.F of formations surrounding the borehole. Still more particularly this invention relates to a method and apparatus for estimating the formation tau, .tau..sub.F, and formation capture cross-section sigma, .SIGMA..sub.F, more precisely through the simultaneous measurement of the thermal decay time constant, .tau..sub.B, and correlative capture cross-section .SIGMA..sub.B of borehole materials.
2. Description of the Prior Art
The technological history of thermal decay time or pulsed-neutron capture logging has been developed in prior patents. U.S. Pat. No. 3,379,882 to Arthur H. Youmans issued Apr. 23, 1963 outlines the physics of pulsed nuclear logging and describes the method of irradiating a formation from a borehole tool with a short burst of fast neutrons and measuring the decline rate of slow neutrons or gamma rays which result from thermal neutron capture in the formation as an indication of the identity of the nuclei of the material present in the formation.
The Youmans patent teaches that the measurement of capture gamma rays is actually more directly indicative of what has occurred in the formation after neutron bursting or pulsing than is a "slow" or thermal neutron measurement. But, if a measurement is made of the entire gamma ray flux produced by the neutron pulse, the initial portion of the gamma ray population curve will depend substantially upon the gamma rays produced by the fast neutron processes, and that the latter portions of the gamma ray population depends entirely upon the slow and thermal neutron processes in the formation. For that reason, as Youmans teaches, the inelastic scattering gammas are preferably distinguished from the capture gammas by initiating the detection interval only after the inelastic gammas may be expected to have substantially disappeared. In other words, the detection interval is preferably started only after the relatively short-lived inelastic scattering gamma rays may be expected to have been dissipated in the earth, and when the thermal neutron population has reached its peak.
Youmans recognized that the lifetime curve of thermal neutrons is a composite of captures occurring in borehole materials, in the porous invaded zone surrounding the borehole, and in the uninvaded formation beyond. Youmans indicated that the preferred method of making a neutron lifetime measurement, for quantitative determination of formation characteristics, is to observe the complete decline curve of the neutron induced radiation (thermal neutrons or capture gammas) from the termination of the neutron pulse to the disappearance of all induced radiation (excluding the activation or background gammas). Thereafter, it is possible to select the portion of the curve having decline characteristics most representative of the formation irradiated, and the other declining portions of the curve will represent the borehole and the borehole substances.
It is generally assumed that the thermal neutrons in the borehole will be captured early, and therefore it is the latter portion of the time cycle which is representative of the formation. However, this assumption is predicated on the requirement that the borehole be filled with substances, such as salt water, which have a thermal neutron capture cross-section greater than either oil or rock substances. If, on the other hand, the borehole is filled with fresh water, oil, or air, the neutron lifetime in the borehole may be much greater than that in the formation material, and it is the earlier portion of the curve which will be representative of the formation. Youmans suggests, then, that it may be desirable to assure that the latter portion of the curve is the portion which is representative of the formation, by the expedient of filling the borehole with salt water (or some other suitable substance) before performing the logging operation.
To restate the foregoing in simpler terms, if the borehole fluids have a higher thermal neutron capture cross-section (and a shorter correlative tau or capture time constant) than that of the formation substances, then the early portion of the curve will be representative of the borehole, and the later portion will be representative of the formation. The second two intervals will best indicate the lifetime of the thermal neutrons in the formations.
U.S. Pat. No. 3,662,179 to Frentrop and Wahl issued May 9, 1972 discloses a pulsed neutron logging system which has seen wide commercial application. Frentrop and Wahl invented a three gate system to measure formation tau, and assumed that by waiting a sufficient time after the neutron burst terminated, the borehole gamma rays induced by neutron capture have died out and that the remaining gamma rays result entirely from formation nuclei capture of thermal neutrons and natural and activation (background) gamma rays.
The Frentrop and Wahl patent assumed that the slope of the mid-portion of the logarithm count of gamma rays is a measure of the formation decay time, .tau..sub.F. The slope or decay time is determined by sampling the count rates during the first two of the three gates. A third gate is positioned after the formation induced gamma rays have died away and is used to measure background. Background is then subtracted from the readings of Gates I and II before computing decay time. The Frentrop and Wahl system provided variable gate width times and starting locations with respect to the end of the neutron burst, yet the relationship of one to another was fixed.
Gate I starts 2T microseconds (.mu.sec) after the end of the neutron burst. Gate I lasts T .mu.sec; Gate II lasts 2T .mu.sec and Gate III is positioned from 6T to 9T. Assuming an exponential decay, (i.e., N=A.sub.e.sup.-t/.tau..sbsp.F), T is adjusted until the counts of the gates, N.sub.1, N.sub.2, N.sub.3 satisfied the equation, EQU 2N.sub.2 -N.sub.1 -N.sub.3 =0.
When the equation is satisfied, adjusting T from cycle to cycle, T=.tau..sub.F.
An improvement in measurement accuracy of formation decay time, .tau..sub.F, is disclosed in U.S. Pat. No. 4,223,218 to Jacobson, issued on Sept. 16, 1980, U.S. Pat. No. 4,224,516 to Johnstone issued on Sept. 23, 1980, U.S. Pat. No. 4,267,447 to Johnstone issued on May 12, 1981, and U.S. Pat. No. 4,292,518 to Johnstone issued on Sept. 29, 1981. The Jacobson and Johnstone system provides sixteen (16) detection gates as distinguished from the three gate system of Frentrop and Wahl. The entire time scale of the neutron generator burst and gate opening and closings is controlled according to a characteristic time, T. But where as in the Frentrop and Wahl system the T is continuously varied to be identical to the measured value of .tau., the Jacobson and Johnstone system adjusts it by surface electronics in discrete steps called F-modes. The variable time - scale, or F-modes, position the gates on the decay curve to most successfully avoid the early casing and borehole signal. Gate I starts approximately 2.tau..sub.F .mu.sec from the end of the minitron burst. The purpose of the time delay between the preceding neutron burst and the beginning of the gating sequence is to permit gamma rays emanating from the immediate borehole environment, (e.g. borehole fluid, casing, cement annulus, tool housing, etc.) to die out before detection of the count rate data from the formation is commenced.
U.S. Pat. No. 4,122,338 to Smith and Pitts issued on Oct. 24, 1978 was an attempt to respond to the problem, recognized as outlined above by Youmans, that, depending on the borehole environment, the gamma ray counts measured after a fixed delay from the end of the neutron burst may include borehole events. In other words, the systems disclosed in the Frentrop and Wahl patent and in the Jacobson and Johnstone patents assume that the borehole thermal decay time, .tau..sub.B, is substantially shorter than the thermal neutron decay time of the earth formation surrounding the borehole. The Smith and Pitts patent indicates that this assumption is generally correct if the borehole is filled with a drilling fluid having a high chlorine or salt water content. However, in boreholes containing air, gas, fresh water or oil base muds, the relationship of .tau..sub.F being significantly longer than .tau..sub.B may not hold. Smith and Pitts measure the actual salinity of the borehole fluids. These measurements are used to control the time delay prior to opening the first measurement gate for detecting .tau..sub.F, thereby assuring that borehole gamma rays are no longer present when gamma rays are counted.
U.S. Pat. No. 4,326,129 to Neufeld issued on Apr. 20, 1982 discloses a method and apparatus for logging boreholes containing air, gas, fresh water or oil e.g., boreholes having a large borehole decay constant, .tau..sub.B, in comparison with the formation decay constant, .tau..sub.F. Neufeld specifies measuring an impulse response function h(t) by using a correlator for autocorrelating the output signals from a detector in response to neutron bursts in the borehole. A correlator for crosscorrelating the output signals from the detector with the signals representing the energy pulses obtained from the source is also disclosed. Neufeld expresses the decaying portion of the impulse response function h(t) as a sum of two component functions exponentially decaying at different rates. These component functions are individually associated with the thermal neutron slowing down processes in the formations surrounding the borehole and in the fluid within the borehole.
Neufeld expresses the impulse response function as h(t)=Ae.sup.-.alpha.t +Be.sup.-.beta.t where A, B, .alpha. and .beta. are constants;
Ae.sup.-.alpha.t represents the neutron population of the formation component; Be.sup.-.beta.t represents the neutron population of the fluid component in the borehole. The constants .alpha. and .beta. represent the thermal neutron decay constants of the earth formation and of the fluid in the borehole respectively. Thus, regardless of the relative values of .alpha. and .beta., Neufeld specifies the measurement of h(t), and then fitting a two exponential model to the measured data to determine .alpha. and .beta.. Neufeld suggests the use of least-squares analysis to determine .alpha. and .beta. of the two exponential model from the measured h(t) data.
A problem has existed in all of the prior work of pulsed neutron logging in that no method and apparatus has been suggested or devised to more precisely measure the borehole decay constant, .tau..sub.B, so that where compensation of the formation decay constant is warranted, a more precise compensation and ultimate value of .tau..sub.F may be determined.